According to the newspaper, dozens of online banking sites slowed, hiccupped or ground to a halt before recovering several minutes later. Security researchers said that instead of exploiting individual computers, the attackers engineered networks of computers in data centers.

“There is no doubt within the US government that Iran is behind these attacks,” said James A. Lewis, a former official in the State and Commerce Departments and a computer security expert at the Center for Strategic and International Studies in Washington.